wait didnt they do that so that they saw the shell splash and would help them find the range and before the last gun fired they could reaim it

All the stuff about hull damage is partly true but only in a few cases of overweight or overarmed ships (e.g. IJN Mogami)

The capsize thing is a complete myth.

The real reason, as I think has been briefly touched on, is aiming. They didn't always fire barrel by barrel. Usually they did a "ladder shot" that involved half or quarter salvos at slightly different ranges, with the time difference allowing the spotters to tell which shell splashes belonged to which shells. That way they could find their range quicker.

Most videos u see of WW2 BBs, especially those that have even number guns in twin turrets (e.g. Hood, Revenge, QE, Bismarck, Nagato etc.), alternately fire one gun from each turret, giving them effectively 2 salvos of 4 guns but twice as often as their theoretical "full" salvo of 8 guns.

BTW, gradual fire can be very useful as an aiming aid in NF especially with slow firing guns, if you haven't learned your angles yet. IRL, they had no way to "learn angles" so they couldn't blockshot from the first salvo =P

most battleship couldnt shoot all guns because of the shear force and recoil behind them

For the smaller ships like DDs and maby CLs if they had 6 inch guns they dont do broadsides simpley because since the shells are so light and the shells are loaded by hand, unlike with the BBs which mashines do half the work, they dont do full broad sides because each crew loads at a different speed meaning that some crews will load their guns before the other ones and if they wait for a broadside there wasting vauluable time which could be spent shooting waiting for the other guns to reload.
This might not apply for the BBs or CAs but for DDs and some smaller gun CLs it is the what I think.

Mainly it was due to ballistics and targeting to improve accuracy. It was much easier and productive to zero in on a target via shell tracking using salvo's instead of full broadsides. Furthermore, most large caliber naval rifles had delay coils installed. Even when a full broadside was fired, each gun was actually fired milliseconds apart. Most triple gunned turrets for the USN was in sequence of gun 1, gun 3, gun 2. This increased accuracy, but the muzzle over-pressures would still interfere with the previous shots ballistics.

The second main reason for salvo's over broadsides was reloading times. If you miss completely with a broadside, your waiting 1 to 2 minutes for the reloading to complete. When firing salvos, if you miss...you zero in and have the next salvo immediately available. In this firing mode, you pretty much had a loaded set at all times.

There was little concern about structural damage. Unless it was a poorly designed ship, they generally were designed to absorb more energy at a single point than the over-pressures of the muzzles were capable of putting out over a larger area. The recoil systems in these gun turrets absorbed most of the energy.